The invention relates to a switching arrangement for a controlled parallel switching of an electrolytic condenser of at least 500 μF capacitance to another, energy-storing electrolytic condenser. The arrangement includes an electronic switch formed by a semiconductor device and a delay member at which the semiconductor device has a control input and the input is connected through an RC-type delay member to a control input which supplies the switching signal. The switching arrangement has a switching delay of determined duration.
Upon suddenly discharging or charging condensers charged with energy, during a given period very large transient currents flow which may damage both the condenser and the switching device that effects the transient process.
In the Hungarian published patent application No. P 9902383 a battery-charging circuit is disclosed which includes a condenser. A switch increases the capacity of the condenser by switching another condenser in parallel therewith. As shown in FIG. 4 of the patent application referred to, a series LC member is inserted into the switching path.
The capacity of the condenser to be charged has a capacity of typically 100-10000 μF, but is advantageously in the 500-10000 μF range. The most wide-spread semiconductor switching device is the MOSFET-type field effect transistor, whose inner resistance is very small in the open state and which may be easily opened and closed at a very high input impedance.
The switching performance of field effect transistors adapted for switching large currents is constantly on the increase, but in any given type currents in excess of a given intensity endanger the soundness of the transistor. In the exemplary use, the maximum permissible current intensity is 180 A. When field effect transistors are used as switching devices, the heat dissipating on the semiconductor device during the transient switching process also constitutes a barrier. In the open state the residual voltage of the field effect transistor is very low, it is typically in the 50 mV range; for this reason the power loss in such a transistor is very small even in case of large currents. During the transient switching process, however, the risks are very substantial that the field effect transistor is exposed to loads greater than the permissible limit values.
Thus, when switching large energies, attention has to be given to loads to which the switching device proper is allowed to be exposed, and also to the maximum load permissible for the switched circuit. The load on the switched circuit may be reduced by delaying the transient process. At the same time, in given applications it is also an object to ensure that the switching itself affects the transient processes appearing in the switched circuit only to the extent as absolutely necessary, that is, up to a prevention of exceeding the limits determined for the components.
Inductive elements are routinely used in the main circuit of the switched circuit for purposes of delay. A problem involved in the use of an inductive element is that the inductivity necessary for effecting the delay can be obtained only with an ohmic resistance of given magnitude, and the presence of the ohmic component in the main circuit causes a continuous loss and adversely affects therein the processes which are no longer transient switching events.
In the uses noted in the above-mentioned Hungarian patent application, boundary conditions develop at which the delay period necessary for the transient process is longer than what the usual electronic semiconductor devices are able to tolerate by way of thermal stress derived from the switching transient in case of currents in the range of 100-200 A. At the same time, such delay period is short enough to render the generally applied inductive delay elements unusable because of the existence of the ohmic component appearing when the required inductivity value is achieved.
Because of the described contradictory requirements, that is, where to a large-capacity electrolytic condenser containing large energy another, energy-less, but also large-capacity electrolytic condenser has to be connected in parallel and where it is a requirement to affect the developing transient processes as little as possible, such task could be solved heretofore only with opening and closing contacts. Solutions utilizing mechanical contacts, however, are disadvantageous as concerns their cost speed, the comfort of control and their low-level reliability as compared to the use of otherwise comfortable, rapid and reliable electronic devices.